Various radar surveillance systems have been proposed using low Earth orbit satellites. One drawback to low earth orbit satellites is that a large constellation is required to provide continuous surveillance coverage. Large low Earth orbit constellations may include dozens of satellites to provide adequate surveillance over a given region of interest. A significant drawback to providing such number of satellites is cost and complexity. To launch and maintain such a system may be cost prohibitive. Also, additional sensor and communications capabilities, due to cost reasons, may be eliminated from the satellite to attempt to reduce these high costs. However, by reducing these features, the flexibility and overall surveillance utility of the system is reduced. Ideally, the problem of persistent surveillance over a region of interest could be accommodated by a few satellites in geostationary orbit. However, the size of the antenna and amount of transmitter power are strong functions of distance to the target being detected and therefore make satellites at geostationary orbits prohibitively large. This is undesirable in many circumstances.
It is desirable that a radar surveillance satellite be able to surveill and track targets almost simultaneously over a wide area within the satellites instantaneous field of regard. A way to achieve this capability is to employ an antenna that allows electronic beam steering, so that swaths hundreds of miles apart may be surveilled almost simultaneously (within a few seconds), and targets hundreds of miles apart may be tracked simultaneously by switching a beam of radar energy back and forth between them every few seconds.
From low Earth orbit, hundreds of miles distance on the ground translates into large angles (tens of degrees) at the satellite, so, for electronic beam steering to work, an electronically-steered phased-array antenna must be used. Such antenna are expensive, and compounded with the need for a large number of them required in low Earth orbit, may contribute to the excessive costliness of a low Earth orbit radar surveillance system.
In medium Earth orbit, the number of satellites needed for a radar surveillance system is drastically reduced. Also the angle over which the radar beam must rapidly steer, in order to simultaneously track targets hundreds of miles apart, is also drastically reduced (to a few degrees).
It would therefore be desirable to provide a satellite system having a constellation design that reduces the number of satellites compared to a low Earth orbit satellite system. It would also be desirable to provide a system that reduces the steering angle needed to simultaneously track targets that are hundreds of miles apart.